U.S. Pat. No. 5,990,926 discloses a projection lens system for use in microlithography and this lens system has three bellied regions, that is, three lens groups of positive refractive power. The objective is viewed in the direction of the propagation of the light. Here, the first lens group includes only positive lenses and the wafer end numerical aperture is 0.6.
U.S. Pat. No. 5,969,803 discloses a projection objective for use in microlithography and this lens system includes three positive lens groups. The numerical aperture again is 0.6 and the objective here is a purely spherical objective.
U.S. Pat. No. 4,948,238 discloses an optical projection system for microlithography wherein, at the wafer end, the last two lenses have respective aspherical lens surfaces for improving imaging quality. The aspherical lens surfaces are arranged facing toward each other.
The projection systems known from the above U.S. Pat. No. 4,948,238 have a low number of lenses. Especially, the numerical aperture, which can be made available by means of this objective, is only 0.45.